This application claims the priority of German application number 196 48 091.4, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a process and apparatus for manufacturing hollow sections with cross-sectional expansions in end portions thereof.
A process and apparatus of the above-mentioned type are disclosed in German Patent Document DE 44 44 759 A1, which provides a process for manufacturing exhaust gas inlet stubs for motor vehicle catalysts. In that process, a pipe-shaped blank is placed in the sinking of a two-part internal high pressure forming tool, and the forming tool is closed. The sinking of each tool part has a projecting wedge-shaped recess which deviates from the axial longitudinal course of the blank shape. Each of the recesses has a uniform construction, but the two are arranged in a mirror-inverted manner, offset with respect to one another at an angle of rotation of 180.degree. about a vertical axis. In the closed position of the forming tool, the blank and simultaneously the sinking are closed in a pressure-tight manner axially by two punches, each provided with a connection for introducing high pressure fluid. During expansion of the blank by the introduction of highly pressurized fluid, the punches push wall material of the blank into the recesses toward the center. The finished blank formed in this case has an asymmetrical construction in the expansion area corresponding to the shape of the sinking.
After the removal of the formed blank, it is divided by a separating device in the expansion area by a planar diagonal cut such that two hollow sections of an identical shape are produced whose ends, which face one another in the separated position, are widened in their cross-sections with respect to the remaining hollow-cylindrical course such that they have a diagonally extending funnel shape. The manufacturing of hollow sections with such high degrees of forming by means of the known process results in a high reject rate. That is, in the case of the construction of small radii at the expansion edges, even on one side of 90.degree., the required very high pressures of above 1,000 bar, and the high forming degrees (the ratio of the diameter of the inserted blank to the largest diameter of the formed component), of above 60%, despite the pushing by way of the follow-up punches, sufficient wall material cannot be supplied into the expansion area. Because of the small quantity of material occurring in this area, the wall of the blank partially becomes so thin, that the blank may burst. The suitable afterflowing of the wall material is prevented by the high friction on the sinking of the wall material pushed in by the punch despite the entering of a lubricant between the sinking and the blank.
It is an object of the invention to provide a process and apparatus of the above-mentioned type by which hollow sections can be produced from a hollow blank with the lowest possible reject rate and therefore in a reliable manner with respect to the process, which by means of high degrees of forming are widened on the end side in their cross-sections.
This and other objects and advantages are achieved by the method and apparatus according to the invention, in which high degrees of forming are permitted in a simple manner by the simultaneous widening by means of a fluid pressure and the axial pressing together of the blank caused by the closing movement of the forming tool which is open at the beginning of the forming operation. With respect to the high fluid pressure (&gt;1,000 bar) required during the forming by a pure expansion, a comparatively low fluid pressure of approximately 200-300 bar must be applied. This also simplifies the fluid pressure generating system because the pressure intensifiers, which are required for very high pressures, are eliminated.
The coordination of the fluid pressure and the upsetting movement takes place such that a danger of buckling can virtually be excluded for the blank. Because of the upsetting, sufficient material is subsequently supplied into the expansion zone so that forming degrees of over 90% will even be possible without the occurrence of crack formations or even a destruction of the blank by bursting.
Since, during the forming process, no relative movement takes place between the blank and the sinking of the forming tool, no friction arises so that the customary lubrication will not be required. This has the result that no problems can occur during the reprocessing of the fluid pressure liquid as a result of filter clogging lubricants. The bulging, rotationally symmetrical blanks formed during the forming process can be easily separated by a simple cut transversely to their longitudinal dimension into two hollow sections.
Because of their rotational symmetry, these are optimally suitable for use as connection stubs for catalyst housings for the conventionally constructed housings, in which case, because of the straight cone of the hollow section according to the invention formed to such a stub, the best possible flow conditions against the catalyst body are ensured. Furthermore, because of the lower demands on a high forming capacity, the process according to the invention permits the use of rust-proof ferritic materials which in contrast to the previously used austenitic materials are significantly lower in price. Because of the lower thermal expansion in comparison to austenitic materials, the use of ferritic materials permits the manufacturing of more compact hollow sections or of assemblies which consist of these hollow sections or contain them.